help converting btrfs to new writeback error tracking?

[Jeff Layton <jlayton@redhat.com>]

I've been working on set of patches to clean up how writeback errors are
tracked and handled in the kernel:

http://marc.info/?l=linux-fsdevel&m=149304074111261&w=2

The basic idea is that rather than having a set of flags that are
cleared whenever they are checked, we have a sequence counter and error
that are tracked on a per-mapping basis, and can then use that sequence
counter to tell whether the error should be reported.

This changes the way that things like filemap_write_and_wait work.
Rather than having to ensure that AS_EIO/AS_ENOSPC are not cleared
inappropriately (and thus losing errors that should be reported), you
can now tell whether there has been a writeback error since a certain
point in time, irrespective of whether anyone else is checking for
errors.

I've been doing some conversions of the existing code to the new scheme,
but btrfs has _really_ complicated error handling. I think it could
probably be simplified with this new scheme, but I could use some help
here.

What I think we probably want to do is to sample the error sequence in
the mapping at well-defined points in time (probably when starting a
transaction?) and then use that to determine whether writeback errors
have occurred since then. Is there anyone in the btrfs community who
could help me here?

Thanks,
-- 
Jeff Layton <jlayton@redhat.com>